Structural preforms are necessary for liquid composite molding (LCM) process. Structural preforms must be formed in the three dimensional shape complimentary of the final desired part. The preform can then be placed in a mold cavity where resin is injected to provide the final form of the part.
The preform is formed by a combination of structural fibers, such as glass fiber reinforcement, and a resin or binder material. A preform must be dense and have little loft to facilitate a high weight percent of glass in the final part. The thickness of the preform must be controlled because the closing action of the mold can tear apart the preform on surfaces with orientations close to vertical. Factors in the cost of a preform include the source of the fibers, i.e., woven mats, continuous strand mats, or chopped roving. Cycle time as well as the amount of scrap generated during each cycle are also factors in cost.
Current process of forming preforms include thermoforming, directed fiber, and slurry methods.
The thermoforming process involves cutting a blank from a glass mat. Blanks are stacked to get the proper amount and orientation of glass, and the stack is passed through an oven and placed in a die cavity. The die is closed to form the glass to the desired three-dimensional shape, and excess material is trimmed. The die is cooled to set the thermoplastic binder in the preform shape. In general, mats are costly and trimming produces significant scrap material. Furthermore, the binder pre-exists within the glass mat and therefore allows no variation in amount or distribution of the binder.
The directed fiber method utilizes a glass chopper aimed at a screen which is shaped as the desired part. A blower draws air through the screen which holds the chopped glass in place. The binder is directed onto the screen along with the glass. The screen is moved into an oven to melt the binder. Exemplary of such process is U.S. Pat. No. 2,929,436 in the name of Hampshire, issued Mar. 22, 1960 which discloses a conventional manual spraying of chopped fibers onto a preforming screen. The problem with the process is that compaction force is limited to atmospheric pressure and has a lessening effect on the outer layers of the preform producing significant loft. Furthermore, uniformity is strictly dependent upon the spray pattern.
The slurry process utilizes a screen which is originally positioned at the bottom of a tank of water. Chopped glass and binder fibers are added to the tank and the mixture is agitated by compressed air. The screen is raised through the tank, filtering all water therethrough and the glass and binder are trapped on the screen. The preform is removed from the screen and placed in an oven to dry the preform. This process is illustrated by U.S. Pat. No. 5,039,465, issued Aug. 13, 1991 in the name of Freeman et al. The problem with this type of system is that additional energy is needed to drive the screen through the water and in an added operation dry the preform. The time of drying can take up to two hours. Furthermore, the ability to produce consistent preforms time after time is unproven, as well as the ability to control the distribution of the glass fiber within a preform.
Other methods have been utilized to form preforms utilizing vacuums. Such is indicated by U.S. Pat. No. 2,725,601, issued Dec. 6, 1955 and U.S. Pat. No. 3,177,275, issued Apr. 6, 1965, both patents in the name of Brenner. In general, chopped fiber is introduced to a chamber and distributed by a fan or rotating blades at the chamber entrance. Binder may be sprayed therein. A partial vacuum is drawn behind the preforming screen to direct the fiber against the screen. The problem with this method is that there is minimal consolidation or compaction of the fibers of the preform, thereby producing significant undesirable loft.
Therefore, it is desirable to use a process which is low in cost, produces high compaction and consistency without the penalties discussed above.